Defect inspection for the DUV range is currently being performed with all-refractive objectives or with catadioptric (combination of refractive and reflective) objectives. For wavelengths shorter than DUV, US Patent publication 2006/0219930 proposes the use of an all-reflective, or catoptric, optical system. US patent publication 2006/0219930 (now U.S. Pat. No. 7,351,980) is hereby incorporated by reference in its entirety.
As design rules shrink in integrated circuit technology, one way to improve the detection of smaller defects is to increase the resolution of the optical inspection system by utilizing shorter wavelengths. However, as the wavelength goes below 250 nm, the dispersion characteristics of the available optical material such as fused silica and CaF2 increases significantly. Furthermore, the availability of manufacturable Anti-Reflective (AR) coating materials effective over the spectrum from sub-200 nm to above 400 nm wavelength is limited. (Good AR coating for a reflective microscope is essential to minimize flare, stray light, etc). These conditions make it extremely difficult to design and manufacture an all-refractive or catadioptric optical system that supports broadband illumination and detection including wavelengths shorter than 250 nm.
The all-reflective optical system proposed in US patent publication 2006/0219930 (now U.S. Pat. No. 7,351,980) provides substantial improvements in manufacturability and design of an optical system for use in wafer inspection with broadband illumination including wavelengths below 250 nm. Furthermore, the design of the objective in U.S. Pat. No. 7,351,980 includes an opening in the mirror adjacent to the wafer that is being inspected. The presence of the opening presents a potential risk of deposition of contaminants onto the wafer and/or diffusion of contaminations into the objective. Such contamination risks are undesirable in wafer inspection systems. One could reduce contamination risk by providing a large flow through the opening that is presented by an all reflective optical design. However, this creates mechanical instabilities at or near the opening. U.S. Pat. No. 7,138,640 describes a method for protecting optical components using a gas purge system that blocks contaminants from reaching the optical surfaces of optical components and transports contaminants away from those surfaces. However, it may be difficult to implement this method in a broadband system that includes wavelengths below 200 nm due to color correction, AR performance issues and or mechanical instability if the window is made too thin.
The prior art describes a number of sample inspection systems that have various other disadvantages. For example, U.S. Pat. No. 6,867,424 describes an optical system in which there is only a single imaging path preventing simultaneous usage of two light sources with different modes.
U.S. Pat. No. 7,359,044 describes the use of laser-based illumination (as opposed to broadband illumination) for bright field and dark field imaging in a sample inspection system. Multiple lasers are used to provide illumination, but there is only a single imaging path.
Lange 20050052643 describes an inspection system in which there are dual illumination paths but only a single imaging path.
U.S. Pat. No. 6,404,498 also describes a system in which there are dual illumination paths but only a single imaging path.
U.S. Pat. No. 6,078,386 teaches an inspection system having dual imaging paths but uses narrow band, e.g., laser illumination and imaging. In addition, the laser beam is introduced to the sample from outside the objective. When two light sources are used, the narrow band illumination is introduced to the sample from outside the objective in a spatially coherent mode.
U.S. Pat. No. 6,762,831 teaches a sample inspection system that uses narrow band illumination with DUV and VUV radiation introduced to a sample through the objective. Illumination through the objective enables spatially incoherent illumination modes. Illumination and imaging are done with light of two different wavelengths generated by two lasers.
It is within this context that embodiments of the present invention arise.